Sucrose fatty acid esters are conventionally prepared by transesterifying a lower alkyl ester of higher fatty acid with sucrose. Since sucrose has eight hydroxyl groups per molecule, the number of fatty acid groups bound to sucrose per molecule (commonly referred to as "degree of substitution" or "D.S.") may vary from 1 to 8. Among them, mono-, di- and tri-esters find use as non-toxic, biodegradable surfactants and are commercially available in large quantities.
Sucrose fatty acid polyesters having a D.S. of at least 4, preferably at least 6 have been reported to be effective in the treatment of hypercholesteremia. See, Chemical & Engineering News, p.26, Dec. 4, 1978; U.S. Pat. No. 4,005,195; U.S. Pat. No. 4,005,196; European Patent Application No. 69,412 (Jan. 12, 1983); and German Patent Publication No. 2,648,551.
The present invention is directed to the production of such sucrose fatty acid polyesters.
Various methods are known for producing sucrose-fatty acid esters. They may be classified into the following three principal types.
In the solvent process, a fatty acid ester is transesterified with sucrose in a common solvent for the fatty acid ester and sucrose such as dimethylformamide or dimethylsulfoxide in the presence of a basic transesterification catalyst. The reaction may be carried out even at a relatively lower temperature, for example, at about 90.degree. C. This process suffers from certain disadvantages that the solvent used is toxic and, therefore, must be completely removed after the reaction. This is possible in practice only with great difficulty.
In the second process generally known as "microemulsion process", a fatty acid ester is dispersed in a solution of sucrose in a solvent such as propylene glycol or water with the aid of an emulsifier such as alkali metal fatty acid soaps to form a microemulsion, and then the solvent is removed from the emulsion. The reaction is carried out in the absence of solvent and the reaction product does not contain any solvent. Great difficulty is also present in this process for removing the solvent while maintaining the microemulsion state.
In the third process, sucrose is directly reacted with a fatty acid ester by heating their mixture. This process is known as "direct process" or "solvent-free process". Since sucrose and fatty acid esters do not have sufficient affinity to each other, the success of this direct process depends on how they are well contacted in the reaction system. To this end, most of known processes employ an alkali metal fatty acid soap either directly added to or formed in situ in the reaction system to produce a homogeneous molten mixture of reactants.
Consequently, the reaction mixture from the microemulsion process or direct process contains a relatively large amount of alkali metal fatty acid soap, since the soap itself is not a reactant and remains unreacted during the transesterification reaction.
A relatively small amount of alkali metal fatty acid soap is unavoidably formed even in the solvent process by the reaction between the fatty acid ester and the transesterification catalyst such as alkali metal hydroxides and carbonates.
Normally, alkali metal fatty acid soaps remaining in the reaction mixture are separated from sucrose fatty acid esters, while their presence may be tolerated in certain uses such as detergents.
Sucrose fatty acid polyesters may be advantageously produced by the microemulsion process or solvent-free process.
U.S. Pat. No. 3,963,699 to Rizzi et al. discloses a process for producing sucrose fatty acid polyesters. According to this process, a mixture of sucrose, a fatty acid lower alkyl ester, an alkali metal fatty acid soap and a basic catalyst is heated in the first step to form a homogeneous melt. Thereafter, excess fatty acid lower alkyl esters are added in the second step to the reaction product of the first step. This process suffers from certain disadvantages in that it requires such basic transesterification catalysts as alkali metals, alloys of alkali metals, alkali metal hydrides or alkali metal alkoxides which are expensive and dangerous in handling. The two step reaction is cumbersome in operation and necessarily requires a prolonged reaction time which can lead to the risk of darkening of the reaction mixture.
Generally, sucrose fatty acid esters having a D.S. of greater than 2 are produced by controlling the molar ratio of fatty acid lower alkyl esters to sucrose. Up to a D.S. of 5, polyesters may be prepared at the ratio of fatty acid esters approximately equal to or slightly excess of theoretical amounts. However, polyesters having a D.S. of greater than 5 require further excess of fatty acid lower alkyl esters. For example polyesters having D.S. of 5.5, 6 and 7 or higher may only be produced at the ratio of fatty acid esters of about 6, 8 and 10 moles per mole of sucrose, respectively. Thus, it is critical for the industrial production of sucrose fatty acid polyesters to minimize the amount of fatty acid lower alkyl esters.
The presence of large amounts of fatty acid lower alkyl esters in the reaction system at one time produces certain unique problems. A reaction system containing a large amount of fatty acid esters is less viscous and thus easily susceptible to phase separation which adversely affects the transesterification reaction. Furthermore, relatively large amounts of low boiling-point byproducts such as methanol are generated and vigorous foaming of reactants takes place during the initial period of the reaction.
It is therefore an object of the present invention to provide a process for producing sucrose fatty acid polyesters in an efficient manner which is free from the above-described disadvantages and which is simple in operation and easy to control.
As previously noted, sucrose fatty acid esters in the reaction product must be separated from impurities mainly consisting of unreacted fatty acid lower alkyl esters and fatty acid alkali metal soaps.
Fatty acid lower alkyl esters may be removed from the reaction product by solvent extraction using a solvent such as methanol in which sucrose fatty acid esters are relatively insoluble and fatty acid lower alkyl esters are soluble. However, this technique requires a large amount of solvent. For example, about 40 times of methanol are used relative to the sucrose fatty acid ester in the previously cited Rizzi et al. patent. This is of course uneconomical and requires a large amount of investment to solvent recovery system and anti-explosion facilities. Additionally, certain amounts of sucrose fatty acid esters dissolving in the solvent are unavoidably wasted.
Japanese Patent Publication No. 28890/1973 discloses a method for selectively extracting sucrose fatty acid esters from the reaction product containing fatty acid alkali metal soaps with acetone. Unfortunately, this method is applicable only to sucrose fatty acid lower esters but not effective to sucrose fatty acid polyesters because the polyesters are not sufficiently soluble in acetone.
Japanese Patent Publication No. 37168/1975 discloses a method for separating fatty acid alkali metal soap from the reaction product containing sucrose fatty acid esters. The method comprises the steps of dissolving or suspending the reaction product in water or a mixture of water and an organic solvent, adding thereto a water-soluble salt of a multivalent metal to form a water-insoluble metallic soap and separating the metallic soap. This method is also inapplicable to sucrose fatty acid polyesters because appreciable amounts of metallic soap are soluble in the sucrose polyesters.
We have found that fatty acid alkali metal soaps may be removed by leaching the reaction product with an aqueous mixture containing about 20% of a lower alkanol which effectively prevents emulsifying of the entire system. Again, the use of organic solvents is uneconomical and must suffer from various disadvantages as previously discussed.
None of known processes permits separation of both fatty acid lower alkyl esters and alkali metal soaps from sucrose fatty acid polyesters simultaneously.
It is therefore a further object of the present invention to provide a process for isolating sucrose fatty acid polyesters from the reaction product containing the polyester, fatty acid lower alkyl esters and alkali metal soaps in an economical and simple manner.
Other objects and advantages of the present invention will become apparent as the description proceeds.